485 
traire incolore,« and the same may be said of the Fseroese material 
and of Prasiola crispa f. terrestris. 
Though Wille (1. c. p. 13) writes with reference to his forma 
submarina: — »Es kann vielleicht Zweifel dariiber herrschen, warum 
sie nicht als selbstandige Art aufgefuhrt werden soli, da sie keinerlei 
Ubergang aufweist zu Prasiola crispa (typische Flachenform), welche 
auch keineswegs mit ihr zusammen vorkommt,« it is quite natural, 
at any rate now that the typical plate-form has also been found, to 
refer it to Prasiola crispa , with which it is very closely allied. The 
fresh-water material from damp roofs in Thorshavn bears consider¬ 
able resemblance to the marine form; as to the differences which 
exist between them I may point out the following:— In the Hormi- 
duz/n-filaments the cells are altogether somewhat longer, being gene¬ 
rally about V 2 — 3 A times as long as broad, but cell-filaments also 
occur in which the cells are as short as those in the salt-water 
specimens. Rhizoids are comparatively rare in the fresh-water 
material, though they may sometimes be met with, as I have shown 
on Plate IX, fig. 2, a, and Gay (1. c. figs. 126—127) also figures some 
which correspond exactly with those in the marine form. The 
reason why the latter form is so amply supplied with rhizoids may 
possibly be explained by the fact that the alga growing, as it very 
often does, in much exposed situations, must attach itself firmly 
to the rock so as to be able to withstand the force of the waves. 
The plate-form (Prasiola - stage) of the marine specimens exactly 
resembles that of the land-form in the arrangement of the cells, 
but it hardly attains to so large a size as in the latter. Lastly, I 
may here point out that the Gagella- stage has not hitherto been 
found in the land-form. I am therefore of opinion that on the 
strength of these differences the forms referred to Prasiola crispa 
ought to be divided into two sub-species, the marine subspec. 
marina , and the land-form, which may be called subspec. terrestris. 
Further, the plant which Batters (1. c. p. 38, tab. VII, fig. 8) 
calls Ulothrix discifera , judging from the specimens which he sent 
to Rosenvinge asking if they could possibly be Gagella , must 
belong to the present species; there are no rhizoids in his figure, 
though the examples of his plant which are here in Copenhagen are 
richly provided with them. Schizogonium radicans Foslie (1. c.) 
likewise belongs to the present species. Foslie’s specimens, of 
which there are two in the museum in Copenhagen, are precisely 
similar to mine. Ulothrix discifera Kjellm (1. c.), on the other hand, 
31 * 
